This invention relates to fuel control of a multi-cylinder internal combustion engine, and more particularly a control for carrying out individual cylinder fuel control based on the output signal of a single exhaust gas oxygen sensor.
Effective emission control of internal combustion engine exhaust gases with a catalytic converter requires precise control of the air/fuel ratio supplied to the engine cylinders. For this purpose, it is customary to install an oxygen sensor in the engine exhaust pipe, and to use the sensor output as a feedback signal for closed-loop fuel control. Generally, the engine exhaust manifold combines the exhaust gases of several individual cylinders at or near its outlet, and a single oxygen sensor is positioned near the outlet of the exhaust manifold. The controller, in turn, obtains an average reading of the oxygen sensor, and controls the fuel supplied to the engine cylinders to maintain the average air/fuel ratio at a desired value. However, the air/fuel ratio in individual engine cylinders can vary considerably depending on engine hardware design variations and fuel injector performance variations, for example. In other words, certain cylinders tend to run rich, while other cylinders tend to run lean, relative to the desired air/fuel ratio. This imbalance limits the precision of the overall fuel, EGR and spark controls, and therefore limits improvements in both exhaust gas emissions and fuel economy.
In view of the above, it has been proposed to individually control the fuel supplied to each engine cylinder; see, for example, the U.S. Pat. No. 5,651,353 to Allston, issued on Jul. 29, 1997. Some systems of this type utilize multiple oxygen sensors for developing air/fuel ratio feedback signals unique to each cylinder. Some systems utilize a single oxygen sensor, but variations in the transport delay time between a cylinder exhaust port and the oxygen sensor make it difficult to correlate the sensor readings with a given cylinder. In certain instances, transport delay variations are empirically determined for a given engine design, and stored as a fueling calibration. In other instances, mathematical models of the engine and exhaust system are used to compute individual air/fuel ratio feedback signals. The computation approach is often impractical due to through-put limitations, and both approaches suffer inaccuracy due to component variation and degradation over time. Accordingly, what is needed is an individual cylinder fuel control that adaptively balances the air/fuel ratio in the various engine cylinders based on the output signal of a exhaust gas single oxygen sensor without imposing an a severe computational burden on the engine controller.
The present invention is directed to an improved internal combustion engine fuel control wherein a single oxygen sensor responsive to the combined exhaust gas flow of a bank of engine cylinders is used both to control the overall or average air/fuel ratio, and to trim the air/fuel ratio in the individual engine cylinders. According to the invention, the oxygen sensor output is sampled in synchronism with the engine firing events, but at twice (or higher) the frequency, and filtered by an engine speed dependent high-pass filter to form a measure of the air/fuel ratio imbalance with respect to time. The imbalance signal, in turn, is parsed into an array of imbalance values that are associated with individual engine cylinders based on engine operating conditions, and the imbalance values are then used to develop correction factors for the respective engine cylinders that reduce the imbalance while preserving the overall or average air/fuel ratio of the engine.